1. Field of the Invention
The present invention relates generally to metrology and more particularly, to interferometric measurement technology embodied in a portable extended life metrology system.
2. The Prior Art
Interferometric measurement technology is useful in a number of fields. For example, interferometric measurement technology is used in the precise measurement of the linear displacement of material components, such as in the manufacture of machined metal parts, optically finished components and semiconductor integrated circuits. In making such displacement measurements, two techniques are currently in wide use: laser interferometry, in which the displacement of a mirror (attached to the part whose displacement is to be measured) is measured with a precision on the order of 1/16-wavelength (about 1.5 microinches); and linear encoders, in which a scale is attached to the moving part, and its position is read out with a precision of the order of 1/8scale division (or about 126 microinches for a 1000 line/inch scale). The ultimate precision of this technique is on the order of 50 to 100 microinches. Laser interferometry is very accurate and precise but requires a stabilized laser, elaborate optical components, and considerable expertise in its application. Linear encoders are robust and simple to use, but are limited in their precision by (a) difficulty in interpolating to better than 1/8-scale division, and (b) difficulty in manufacturing and reading out very fine scales (e.g. more than 2000 lines/inch).
In U.S. Pat. No. 5,098,190 that issued to Geert J. Wyntjes et al. on Mar. 24, 1992, a metrology system is disclosed that includes means for generating an interference fringe pattern as a function of a parameter to be measured, transducer apparatus for simultaneously generating three intensity-modulated optical signals, I.sub.R, I.sub.S and I.sub.T, that are related to the interference fringe pattern; signal processing apparatus for accurately determining an aspect of the interference fringe pattern from the three signals; means for accumulating phase information proportional to the aspect of the interference fringe pattern; and means for converting the accumulated phase and aspect information to desired outputs indicative of the parameter to be measured. The disclosure of said U.S. Pat. No. 5,098,190 is incorporated herein by reference.
As stated therein, transducer generated optical intensity signals, I.sub.R, I.sub.S and I.sub.T, which under ideal circumstances differ only in their phases .PHI., are expressed by the following relationships: EQU I.sub.R =I.sub.1 +I.sub.2 cos (.PHI.-2.pi./3), (1) EQU I.sub.S =I.sub.1 +I.sub.2 cos (.PHI.), and EQU I.sub.T =I.sub.1 +I.sub.2 cos (.PHI.+2.pi./3),
from which the instantaneous value of the parameter of interest (such as the displacement of a scale relative to the transducer apparatus can be uniquely determined. The specific relationship between scale position, x, and phase, .PHI., (the phase angle .PHI. carrying the basic interferometric measurement information, which in a typical case is lateral displacement, x) being given by: EQU .PHI.=.PHI..sub.0 +2.pi.x/d radians, (2)
where d is the separation between adjacent rulings on the scale, and .PHI..sub.0 is a constant. In terms of R, S, and T (the electrical signals corresponding to the optical intensity signals I.sub.R, I.sub.S, and I.sub.T, respectively), ratios A, B, and C are defined by: EQU A=(R-S)/(T-S) (3)
B=(S-T)/(R-T), and EQU C=(T-R)/(S-R).
Concerning these ratios, two factors are immediately obvious: (a) the ratios are independent of the DC light level, I.sub.1 (since both numerators and denominators are differences between pairs of signals), and (2) they are independent of the AC- amplitude, I.sub.2. In fact, combining equations (1) and (2), we find that the ratios are simple functions of the phase .PHI. alone: ##EQU1##
Metrology systems are useful in numerous fields, including lateral and angular displacement applications, velocity measurements, surface profile and contour measurements, and in applications involving the measurement of electric or magnetic field strengths.
In the said U.S. Pat. No. 5,098,190 the source of power, a regulated 5 VDC, was obtained from and constantly being supplied by an AC/DC converter system plugged into a 120 VAC outlet.
In the several applications mentioned above, a portability feature for the metrology system becomes highly desirable. Such portable feature is possible by the use of a battery. Commercially available batteries to provide the required 5 VDC power supply for the metrology system disclosed in said U.S. Pat. No. 5,098,190 have proven to last for about 6 hours of continuous operation. Thereafter, either the battery must be replaced, recharged or, as a practical matter due to the delicate nature of the instrument embodying the metrology system, the instrument must be replaced with a new one.
A much more desirable alternative is to provide a portable extended life metrology system having an extended useful battery life, a system that is capable of useful life for about one year.